Heretofore, there have been known electric power steering devices of the type that an assist force by an electric motor is applied to a steering mechanism for reducing the steering force to be exerted on a steering wheel (hereafter referred to as “handle”). Various controls in the electric power steering device utilize the absolute steering angle (the position indicating the angle from a neutral position) of the handle rotatable through a limited number of rotations which is more than one. The “neutral position” means the position which the handle takes when tires are steered to direct the vehicle straight-ahead.
A rotational angle sensor is generally known as a sensor for detecting the absolute steering angle of the handle. The rotational angle sensor is composed of a slit disc with plural slits which is provided to rotate bodily with the handle and three sets of photo interrupters provided on a steering column in a fixed state.
Of the three sets of photo interrupters, two sets of photo interrupters are provided for detecting the rotational amount and the rotational direction of the slit disc, and the remaining one set of photo interrupters is provided for detecting a neutral position within every one of rotations of the handle. Output signals from these are used for controlling the vehicle.
The range over which the handle is manually rotatable is generally not within one rotation (360 degrees) and for example, extends ±720 degrees (four turns) including 2 rotations (720 degrees) in the leftward direction and 2 rotations (720 degrees) in the rightward direction. However, when an output is given from the one set of photo interrupters for neutral position detection of the foregoing rotational angle sensor, it is uncertain which rotation the output indicates of the four rotations. Thus, it is unable to detect a precise absolute steering angle based on the output signal only from the rotational angle sensor. For this reason, various means have been employed for detecting the absolute steering angle by the use of the output signal from the rotational angle sensor.
Further, recently, cost reduction has been required in detecting the absolute steering angle of the handle. However, in the prior art, a problem has arisen in that in addition to the rotational angle sensor of the aforementioned complicated configuration, a device has to be further provided for detecting the ordinal number of a rotation the handle is making.
An electric motor of an electric power steering device is provided with a resolver as a device for detecting the rotation of the motor and is also provided with a torque sensor as a device for detecting the steering torque of the handle. Since each of these outputs a rotational angle signal, it may be conceivable to utilize the signals for the purpose of cost reduction. However, where an attempt is made to detect the absolute steering angle by the use of the resolver signals of the electric motor, it is uncertain which angular position a rotor of the resolver takes within one revolution or frequency of the electrical angle. This makes it difficult to identify the absolute steering angle of the handle.
Further, the resolver of the torque sensor is of the nature that detects the torsion of a torsion bar which is provided between an input shaft coupled to the handle and an output shaft, and because the resolver of the torque sensor also outputs plural signals, per rotation of the handle, it is difficult to identify the absolute rotational angle of the handle. Therefore, it has been unable to detect the absolute steering angle of the handle even by individually using the signals output from the resolvers of the electric motor and the torque sensor.
Moreover, there is known an absolute steering angle detection method for a handle which is described in, for example, Japanese patent application No. 2001-268388 (hereafter referred to as “prior art”). According to the absolute steering angle detection method for a handle, a first detection signal is linearly output in dependence on the rotational angle of the handle from first detection means for torque detection which detects the steering torque of the handle. A second detection signal different in frequency from that of the first detection means is linearly output from second detection means for detecting the rotation of a motor which is driven in dependence on the rotation of the handle to assist the rotation of the handle. An absolute steering angle is calculated by calculation means based on the difference between the first and second detection signals and also based on a reference difference per rotation of the handle between the both detection signals.
However, in the aforementioned prior art, a problem remains unsolved in that the absolute steering angle cannot be detected precisely for the reason that there are not taken into consideration any error in steering angle precision of the resolver of the torque sensor, the looseness between gears connecting the handle with the electric motor, any error in electrical angle absolute precision of the resolver of the electric motor, and the like.
Accordingly, the present invention is made in view of the unsolved problem in the prior art, and it is an object of the present invention to provide an absolute steering angle detection device and an absolute steering angle detection method for an electric power steering device which are capable of precisely detecting the absolute steering angle even in the existence of the error in steering angle precision of the resolver of the torque sensor, the looseness between gears connecting the handle with the electric motor, the error in electrical angle absolute precision of the resolver of the electric motor, and the like.